This relates to a chain-type device for machining an outer surface of an object, in particular of an approximately cylindrical object such as a pipe. In one particular aspect, this relates to a scraper chain for tubular objects, in particular plastic pipes.
In many cases, the surfaces of pipes, cables or cylindrical rods must be machined before further treatment. In the case of cables, for example, it is necessary to remove an insulating layer, whereas when heat welding plastic pipes, the surface of the pipes must first be scraped prior to welding. A layer in the order of approximately 0.05 mm to 0.3 mm (or deeper for pipes with a larger bore) is removed from the areas to be welded together. This removes any trapped dirt from the production process and any age-related material changes that may arise from UV damage or surface weathering, for example, which would otherwise result in welds of low quality. Plastic piping on rolls has a certain degree of ovality resulting from the production process, which can also occur in straight plastic pipes due to long periods in storage.
A machining device in the form of a chain, also called a scraper chain, is known in practice, therefore, and is shown in FIGS. 1 and 2. FIG. 1 shows a schematic oblique view of such a conventional scraper chain. The scraper chain has a chain link 20 with a tool holder 10 and a plurality of chain links 60 as chain connecting links arranged like a ring around an object to be machined, such as a pipe, and can be tensioned by means of a tensioning means 80, 82 so that the tool holder 10 lies securely on the lateral surface. A scraper tool 30, for example a scraper blade or a planing tool which removes material from the surface of the object with an adjustable swarf thickness, is attached to the tool holder. In use, the scraper chain is manually moved circumferentially around the object by means of handles 90 attached to the chain, such that the lateral surface is removed substantially annularly or helically. As shown in FIG. 1, the tensioning means may have a tensioning strap 80 and a strap lock 82, but a spring device 84 may optionally be provided that elastically adjusts the length of the chain and facilitates adjustment of the chain tension to an optimal value. A tension gauge, not shown, may be provided on the spring device 84, for example, in order to indicate the chain tension.
FIG. 2 shows a schematic plan view of the underside of the tool holder 10 of the conventional scraper chain in FIG. 1. As shown in FIG. 2, the chain links 60 have a plurality of support rollers 70 arranged in pairs, with which they roll on the lateral surface of the object to be machined. The tool holder 10 has feed rollers 40, preferably two pairs of feed rollers. The axes of the feed rollers 40 can be adjusted by means of adjusting screws 50, 51 relative to the tool holder 10 and thus in relation to the pipe axis by an adjustable angle α, so as to roll at an angle to the scraper chain tensioned around a circumference of the lateral surface and thus to gradually move the scraper chain forward a bit along the pipe axis during a revolution. To do so, the feed rollers 40 have a surface with increased adhesiveness, for example a serrated surface, in order to forcibly cause a feed motion and not slide over the lateral surface at an angle to the axis of the support rollers. In this manner, the tool 30 is guided in a helical line around the lateral surface of the pipe, so as to machine the lateral surface without gaps when the feed rate is correctly set. To achieve this, the feed motion must be set in such a way that it is less than the cutting width of the tool 30, and preferably less than ¾ of the cutting width. Adjusting the feed motion by pivoting the axes of the feed rollers requires a great deal of experience on the part of the operator and is time-consuming due to the feed rate having to be checked and frequently readjusted. The feed rate is also limited to a relatively small amount, as the pivot range of the feed roller axes is small and because the scraper chain tensioned around the object to be machined remains aligned in the circumferential direction with its support rollers 70, which means that the support rollers 70 must slide at an angle over the lateral surface due to the feed motion in the axial direction, which in turn inhibits the feed motion in the axial direction.